1. Technical Field
The present invention relates to a frequency measuring apparatus or the like, and in particular to improvement and so on of a frequency measuring apparatus for making it possible to measure an absolute frequency using a frequency measuring apparatus of a short gate time count method for counting an input signal in a predetermined time period, and removing a high-frequency component from a counted value train, thereby detecting a frequency variation component.
2. Related Art
As a method of measuring a frequency, there are known a direct count method (see, e.g., JP-A-2001-119291) of counting pulses passing therethrough in a predetermined gate time, a reciprocal method (see, e.g., JP-A-5-172861) of accurately measuring the pulse period and obtaining the frequency using the inverse of the period thus measured, and a method (see, e.g., U.S. Pat. No. 7,230,458) of obtaining a Δε modulation signal to thereby find out the frequency.
The applicant has conducted research and development on a frequency measuring apparatus using a short gate time count method.
The frequency count method is arranged to repeatedly perform counting (sampling) with a short gate time without interruption to thereby remove a high-frequency component from the count value train thus obtained, and is capable of significantly improving both of the time resolution and the frequency resolution. The frequency counter of the present method is composed of a counter circuit and a small-scale arithmetic circuit, and has an advantage of easily achieving a multi-channel configuration while suppressing growth of the circuit scale. Further, it has an advantage that the resolution is improved as the sampling frequency is raised, for example.
However, the value obtained by demodulating the pulse train obtained by the short gate time count method is provided as a relative value of frequency. Although the counter circuit can be simplified, and it is sufficient for observing a relative variation, information of the absolute value (entire digits) of the measured frequency is missing.
Further, in the case of using only the least significant bit of the counter as the short gate time count value, it is not achievable to determine based only on the 1-bit information whether the train of 0 and 1 output from the counter positively corresponds to the magnitude of the count value or they have a complementary relationship.